The Apache HTTP Server is designed to be a powerful and
flexible web server that can work on a very wide variety of
platforms in a range of different environments. Different
platforms and different environments often require different
features, or may have different ways of implementing the same
feature most efficiently. Apache httpd has always accommodated a wide
variety of environments through its modular design. This design
allows the webmaster to choose which features will be included
in the server by selecting which modules to load either at
compile-time or at run-time.

Apache HTTP Server 2.0 extends this modular design to the most basic
functions of a web server. The server ships with a selection of
Multi-Processing Modules (MPMs) which are responsible for
binding to network ports on the machine, accepting requests,
and dispatching children to handle the requests.

Extending the modular design to this level of the server
allows two important benefits:

Apache httpd can more cleanly and efficiently support a wide
variety of operating systems. In particular, the Windows
version of the server is now much more efficient, since
mpm_winnt can use native
networking features in place of the POSIX layer used in
Apache httpd 1.3. This benefit also extends to other operating
systems that implement specialized MPMs.

The server can be better customized for the needs of the
particular site. For example, sites that need a great deal of
scalability can choose to use a threaded MPM like
worker or event, while sites requiring
stability or compatibility with older software can use a
prefork.

At the user level, MPMs appear much like other Apache httpd
modules. The main difference is that one and only one MPM must
be loaded into the server at any time. The list of available
MPMs appears on the module index page.

On Unix and similar platforms, MPMs can be built as DSO modules and
dynamically loaded into the server in the same manner as other DSO
modules. Building MPMs as DSO modules allows the MPM to be changed by
updating the LoadModule directive
for the MPM instead of by rebuilding the server.

LoadModule mpm_prefork_module modules/mod_mpm_prefork.so

Attempting to LoadModule
more than one MPM will result in a startup failure with the
following error.

AH00534: httpd: Configuration error: More than one MPM
loaded.

This feature is enabled using the
--enable-mpms-shared option of the configure
script.
With argument all, all possible MPMs for the platform
will be installed. Alternately, a list of MPMs can be specified as the
argument.

The default MPM, either selected automatically or specified with the
--with-mpm option of the configure
script, will be loaded in the generated server configuration file. Edit the
LoadModule directive to select a
different MPM.

Notice:This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our mailing lists.